Report an SkSL error if an in var has an initializer expression.

This resolves the fuzzer error, as the program will fail compilation
before reaching the SPIR-V translation stage at all.

Change-Id: Ia73af497b1f57314a29878f2d2a29dc80186e630
Bug: oss-fuzz:27300
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/333130
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

Author: johnstiles-google

src/sksl/SkSLIRGenerator.cpp

@@ -395,7 +395,7 @@ StatementArray IRGenerator::convertVarDeclarations(const ASTNode& decls,
                 }
                 String name(type->name());
                 int64_t count;
-                if (size->kind() == Expression::Kind::kIntLiteral) {
+                if (size->is<IntLiteral>()) {
                     count = size->as<IntLiteral>().value();
                     if (count <= 0) {
                         fErrors.error(size->fOffset, "array size must be positive");
@@ -428,6 +428,9 @@ StatementArray IRGenerator::convertVarDeclarations(const ASTNode& decls,
             if (!value) {
                 return {};
             }
+            if (modifiers.fFlags & Modifiers::kIn_Flag) {
+                fErrors.error(value->fOffset, "'in' variables cannot use initializer expressions");
+            }
             value = this->coerce(std::move(value), *type);
             if (!value) {
                 return {};

tests/sksl/errors/golden/InVarWithInitializerExpression.glsl

@@ -1,3 +1,4 @@
+### Compilation failed:
 
-in float x = 1.0;
-in float z = y = 1.0;
+error: 1: 'in' variables cannot use initializer expressions
+1 error